Electricity is supplied to conventional electrical devices after they are connected to a socket through a plug and form an electrical connection, so that the electrical devices can operate. If high temperature conditions occur in common electrical devices due to consuming relatively large amount of power or use for a long time etc. reasons, these devices may have overheating protection systems installed, so that these devices would be protected against burning down by means of these protection systems. Moreover, all of the indoor power networks are connected to a circuit breaker (also known as power master switch) that would perform on-off controls. In case the amount of electric current is larger than the preset value, the system would automatically form a short circuit or a fuse would be installed within the circuit breaker, so that when the amount of electric current is larger than the preset value, the fuse would blow out and thus open a circuit to form a short circuit and protect the electric circuit from getting damaged.
The electric wires that catch fire are one of the reasons leading to fire disasters in old-fashioned buildings, and the most common situation in which the electric wires catch fire is the case wherein the circuit is old and has low load capacity and thus become overloaded. When the electric current surpasses the ampere-capacity of the electric wire, the heat energy of the core wire exceeds the heat energy that can be carried, and if it reaches above the allowed electric current, it would possibly ignite the surroundings or cause a spark. If there is a flammable substance, for instance a blanket, window curtains, wallpapers etc., around the electric wire, these materials can easily catch fire and burn, and as a result, cause serious casualties.
Since the rate of aging for circuits are not same, usually, the circuit found within the socket that contacts with air most frequently is the one that gets old quickest. The lower electric current capacity of the circuit that gets old and the preset circuit capacity may not be the same, and therefore after the circuit gets old to a certain degree, the preset electric current throughput of the fuse found on the circuit breaker would be greater than the electric current throughput that the old circuit can carry. As a result, the fuse can not respond when the old circuit is overloaded, and thus the electric wire catches fire.